Dimensionality Control of Inorganic and Hybrid Perovskite Nanocrystals by Reaction Temperature: From No‐Confinement to 3D and 1D Quantum Confinement

Abstract This work focuses on the systematic investigation of the shape, size, and composition‐controlled synthesis of perovskite nanocrystals (NCs) under inert gas‐free conditions and using pre‐synthesized precursor stock solutions. In the case of CsPbBr3 NCs, we find that the lowering of reaction temperature from ∼175 to 100 °C initially leads to a change of morphology from bulk‐like 3D nanocubes to 0D nanocubes with 3D‐quantum confinement, while at temperatures below 100 °C the reaction yields 2D nanoplatelets (NPls) with 1D‐quantum confinement. However, to our surprise, at higher temperatures (∼215 °C), the reaction yields CsPbBr3 hexapod NCs, which have been rarely reported. The synthesis is scalable, and their halide composition is tunable by simply using different combinations of precursor solutions. The versatility of the synthesis is demonstrated by applying it to relatively less explored shape‐controlled synthesis of FAPbBr3 NCs. Despite the synthesis carried out in the air, both the inorganic and hybrid perovskite NCs exhibit nearly‐narrow emission without applying any size‐selective separation, and it is precisely tunable by controlling the reaction temperature.


Introduction
Colloidal lead halide perovskite nanocrystals (LHP NCs) have recently emerged as an ew class of light-emitting semiconductors having ag reat potential for aw ide range of potential applications owing to their interesting optical and electronic properties. [1] In particular, the high photoluminescence quantum yield and the tunable emission across the visible spectrum of light make them attractive light sources for awide range of applications such as light-emitting devices (LEDs), displays,a nd lasers. [1f, k, 2] Theo ptical properties of LHP NCs are easily tunable by their halide (Cl, Br,a nd I) composition as well as by their dimensions. [1a,d,3] Forinstance, 2D nanoplatelets (NPls) and 0D nanocubes of LHPs exhibit strong quantum confinement effects and the optical properties are tunable by their thickness and size,r espectively. [1a, d, 3a, 4] The3 D, 2D,a nd 0D refer to the free movement of electrons in 3, 2, and 0d imensions,r espectively,i no ther words,t he electrons are confined in 0, 1a nd 3d imensions, respectively.F or instance,3Dnanocubes exhibit no quantum confinement in 3d imensions and show bulk-like optical properties.O nt he other hand, 0D nanocubes exhibit 3Dquantum confinement, while the 2D NPls exhibit 1Dquantum confinement. Thea nother important feature of LHP NCs is that they exhibit higher stability compared to bulk thin-film perovskites due to their surface protection by capping ligands.T his has opened doors for the fabrication of relatively stable perovskite solar cells using LHP NCs. [5] The demand for LHP NCs in many potential technological applications has motivated chemists toward the development of facile synthesis methods. [1b,i,3c, 4a,6] Over the years,various strategies such as hot-injection, [1b] ligand-assisted reprecipitation (LARP), [1h, 6c] ultrasonication, [3c, 7] solvothermal, [8] microwave, [9] and ball milling [10] have been exploited for the shape-controlled synthesis of LHP NCs.A mong all, LARP and hot-injection synthesis are the most widely used approaches for both organic-inorganic hybrid and all-inorganic perovskite NCs.Although the LARP approach is well established for monodisperse nanocubes, [6a] it often yields polydisperse NPls and nanowires. [11] On the other hand, the hot-injection synthesis offers better shape control and size distribution of LHP NCs. [1a, b, 4a, 12] This approach is generally based on the injection of ap re-synthesized monovalent cation precursor (for example,c esium-oleate for allinorganic LHP NCs) into af reshly prepared lead halide precursor solution at high temperature and under inert conditions. [1b] In fact, the synthesis of monovalent cation precursors has also been generally carried out under an inert atmosphere.A lthough hot-injection synthesis has proven to be an excellent approach for the synthesis of LHP NCs,i ti s laborious and requires inert conditions.T he first question is: can we synthesize high-quality colloidal perovskite NCs with shape and size control under inert gas-free conditions?O n the other hand, the LHP NCs prepared by hot-injection synthesis exhibit blue-shifted emission with ad ecrease in reaction temperature.T his was attributed differently (either to decrease in the size of nanocubes or to the formation of nanoplatelets) in different reports. [1b, 4a] Therefore,t he morphology and the PL emission of corresponding NCs obtained at different temperatures is not fully understood. To address these questions,h erein we systematically investigated the synthesis of colloidal LHP NCs in the air using presynthesized precursor stock solutions at different reaction temperatures.W ef ind that the decrease of reaction temperature from % 175 to 100 8 8Cleads change in morphology from bulk-like 3D nanocubs to strongly confined 0D nanocubes, while at temperatures below 100 8 8C2 DN Pls with are obtained. However,a tv ery higher temperatures (> 200 8 8C), the reaction yields hexapod NCs in the case of CsPbBr 3 .Such hexapods NCs were previously reported through amulti-step process.The optical properties of the resultant LHP NCs can be precisely tailored through the control of the reaction temperature.I na ddition, the halide composition of the NCs and thus their emission color is varied by simply using different ratios of corresponding pre-synthesized precursor solutions.The synthesis is also applicable to organic-inorganic hybrid perovskite NCs.F or instance,w eh ave demonstrated the synthesis of FAPbBr 3 nanocubes of different sizes and nanoplatelets of different thicknesses by controlling the reaction temperature.T he prepared FAPbBr 3 NCs exhibit narrow emission with as ingle PL peak suggesting the shape purify of the prepared NCs.T he synthesis of perovskite NCs on ahot-plate is illustrated in Figure 1a.This method is based on the simple addition of aCs-oleate solution to aPbX 2 (X = Cl, Br,orI)precursor solution at acertain temperature.T he stock solutions of precursors are prepared by dissolving their respective salts in octadecene with the help of ligands (oleylamine and oleic acid) at 125 8 8Cu nder atmospheric conditions (see the experimental section in the Supporting Information for detailed description). Thep recursor stock solutions are stable for more than 6months and can be readily used to prepare perovskite NCs in large quantities.T his hotplate approach is inspired by the well-studied hot-injection synthesis of perovskite NCs, [1b] however, the pre-synthesized stock solutions and the synthesis under atmospheric conditions make this approach faster, user-friendly,scalable,and more practical for device applications.

Results and Discussion
We first studied the influence of the temperature in the synthesis of CsPbBr 3 NCs.T hus,ap re-heated Cs-oleate precursor solution was injected into aP bBr 2 precursor solution at 175 8 8Co r2 15 8 8Cu nder vigorous stirring (see Figure 1a and the experimental section in Supporting Information for more details). In both experiments,t he colorless reaction medium immediately turns yellow and exhibits intense green emission under UV light illumination, indicating the formation of CsPbBr 3 NCs.A lthough initial studies claimed that CsPbBr 3 NCs exhibit cubic crystal structure, recently it has been widely accepted that they crystalize in the orthorhombic phase,a si llustrated in Figure 1b. [6b, d] As depicted in Figure 1c,t he purified colloidal solutions in hexane exhibit absorption onsets at~510 and 515 nm with narrow emission and symmetrical PL peaks at~515 and 520 nm for samples synthesized at 175 8 8Ca nd 215 8 8C, respectively.T he absorption and emission peaks are typical for 3D CsPbBr 3 NCs that exhibit bulk like optical properties.T he shape and size distribution of the CsPbBr 3 colloidal NCs is studied by transmission electron microscopy (TEM). The TEM images (Figure 1d and 1e) shows that the CsPbBr 3 NCs prepared at 175 8 8Ca re nearly-monodisperse nanocubes with an average edge length of 9.3 AE 1.1 nm, while the NCs obtained at 215 8 8Ce xhibit hexapod morphology with sizẽ 20 nm (also see Figure S1 for HAAD-STEM images of hexapods). In both the cases,t he size of CsPbBr 3 NCs is higher than their exciton Bohr radius (~7nm) and therefore they exhibit bulk-like optical properties. [1b] Thep hotoluminescence quantum yields (PLQYs) for nanocubes and hexapods were measured to be 95 %a nd 5%,r espectively (see Table S2 in SI). Thelow PLQY of hexapods could be due to the presence of defects that needs further in-depth spectroscopic investigation. However,t he monodispersity of the CsPbBr 3 nanocubes with as tandard deviation of~10 % and the PLQY of~95 %obtained by this hot-plate approach seems to be similar to those obtained by classical hot-injection synthesis under degassed and inert conditions. [1b,4a, 6b,13] The crystal phase of the obtained nanocubes and hexapods were characterized by powder X-Ray diffraction ( Figure S3), and the pattern resembles the orthorhombic phase according to the literature. [14] Thes calability of this approach is demonstrated by increasing the volume of the two precursors in the reaction medium by 25 times to obtain 160 mL of CsPbBr 3 NC colloidal solution in as ingle run, with the same concentration as in small-scale synthesis.T he CsPbBr 3 NCs thus obtained exhibit the optical features and morphology (nanocubes of~9.3 nm) similar to those prepared in small-scale synthesis (see Figure S2). This makes the hot-plate approach promising for obtaining perovskite NCs for industrial-scale device applications in the future.
On the other hand, the hexapods obtained at 215 8 8Ca lso exhibit excellent monodispersity with an increase in size compared to nanocubes.The increase of size clearly reflects in the red-shifted absorption and emission spectra of hexapods. In fact, the formation of hexapods at higher temperatures has come as surprise to us.Such multi-faceted hexapod perovskite NCs have been very rarely reported in the literature. [1a, 14] For instance,Peng et al. [14] reported the synthesis of such hexapod NCs by hot injection synthesis using pre-synthesized and purified CsPbBr 3 nanocubes as seeds and oleylamine-hydrohalic acid complex. It was claimed the seeded growth of hexapods occurs through intermediate polyhedron-shaped NCs under halide-deficient conditions.I nt he present work, similar hexapod NCs are obtained in ao ne-pot hot-plate approach. However,t he growth mechanism is currently unclear. Ther esults clearly demonstrate the importance of reaction temperature on the morphology of the resulting perovskite NCs.T herefore,w eh ave systematically investigated the influence of reaction temperature on the morphology and optical properties of the resultant CsPbBr 3 perovskite NCs by performing the synthesis under different temperatures ranging from 150 to 50 8 8C ( Figure 2). Figure 2a shows that the emission color of the resulting colloidal dispersions shows ac lear shift from green to deep blue and the corresponding PL spectra show ag radual shift in the photoluminescence from 516 nm to 455 nm with decreasing the reaction temperature ( Figure 2a). Previously,Alivasatos and co-workers [4a] reported such blue-shifted peaks of CsPbBr 3 NCs for NPls of different thicknesses,w hile Kovalenko and co-workers [1b] reported for CsPbBr 3 nanocubes of different edge lengths.I nb oth reports,t he NCs were prepared by classical hot-injection synthesis at different reaction temperatures.
Therefore,h erein, we carried out TEM analysis of our samples to understand the blue-shifted emission in this work (Figure 2b-d &S 4-S6 (Supporting Information)). TheP L emission peak, PLQY,a nd morphology of NCs obtained at different reaction temperatures are summarized in Table S2 in the supporting information. Interestingly,wefind that that the morphology NCs obtained in the temperature range of 175-125 8 8Cr emains cubic with ad ecrease of their average edge length from % 10 nm to 5.7 nm, and they all exhibit over 90 %PLQY (see Figures 1d,2b&S4f or 175, 150, and 125 8 8C, respectively,a nd Table S2). However,f urther decreasing the reaction temperature to 100 8 8Cleads to the formation of NPls together with nanocubes ( Figure 2c,a nd see Figure S5 for large-area images). Based on the TEM images,the estimated thickness of the NPls and the size of nanocubes obtained arẽ 1.9 nm (3-monolayer-thick) and~3.7 nm, respectively ( Figure S5). Thes ize of the nanocubes is well-below the exciton Bohr radius (~7nm), thus they exhibit 3D-quantum confinement. [1b] Interestingly,at508 8Cmonodisperse 3-monolayer-thick NPls with 1D-quantum confined are obtained (Figures 2d &S 6), and they exhibit orthorhombic crystal phase similar to nanocubes ( Figure S3). [15] These results suggest that, in our case,the initial blue shift in the PL peaks from~515 nm to~470 nm is due to the decrease of the size of nanocubes,w hile the further blue shift down to 455 nm is caused by the formation of 3-monolayer-thick NPls.T he PL spectra of the NPls show one main peak with asmall shoulder peak and this indicates that the colloidal solution contains nearly-monodisperse NPls of single thickness with only avery little contribution of other thicknesses.H owever,t he NPls exhibit low PLQY (5 %) compared to that of nanocubes (95 %). Thel ow PLQY of NPls is due to large number of defects caused by high surface area, and is well known from literature that they require post-synthetic surface passivation to achieve high PLQY. [15] Furthermore,w ed emonstrate that the synthesis of CsPbBr 3 NPls also scalable similar to that of CsPbBr 3 nanocubes by increasing the volume of the precursors in the reaction medium by 20 times to obtain af inal volume of 130 mL with the same thickness of NPls obtained in small-scale synthesis (See the characterization of NPls in Figures S6 for small-scale and S7 for large-scale). However, we find that the strongly quantum confined perovskite nanocubes and NPls exhibit relatively poor long-term stability compared to 3D nanocubes,a si tw as previously reported. [3b] Overall, regardless of the morphology,i ti sc lear that the PL peak position of CsPbBr 3 NCs can be precisely tuned across from blue to green by controlling the reaction temperature. Figure 2eillustrates the PL energy of the NCs obtained at different temperatures vs.t heir size/thickness.O ne can clearly see anonlinear increase in the energy of the PL peak as the size/thickness gets smaller.T he increase in the PL energy is due to the decrees of the size of nanocubes,while the nonlinear increase caused by the formation of NPls at reaction temperatures below 125 8 8C. Therefore,o ne should carefully assign the blue shifted emission of perovskite NCs either to 0D nanocubes or 2D NPls.I na ddition, we demonstrate that this hot-plate approach is also applicable to tune the optical properties of CsPbI 3 NCs by varying the reaction temperature ( Figure S8 in the Supporting Information). However,the PL spectra of the NCs obtained at lower reaction temperatures ( 80 8 8C) are broad due to the presence of NPls of different thicknesses. Furthermore,w ed emonstrate that this approach is generally applicable to the synthesis of halide perovskite NCs of any halide composition. Figure 3a,billustrates the synthesis of CsPbX 3 NCs (X = Cl, Br, I, or their mixtures) of different halide compositions using corresponding combinations of PbX 2 precursor stock solutions along with Cs-oleate. In at ypical synthesis,C s-oleate was added into the desired halide precursor or am ixture of different halide precursors (PbCl 2 /PbBr 2 or PbBr 2 /PbI 2 )at175 8 8C on ahot-plate followed by cooling the reaction mixture in an ice bath (see the experimental section in Supporting Information for details). Ther esulting colloidal dispersions of CsPbX 3 NCs show different light emission upon UV illumination, covering the entire visible range,w hich indicates different halide compositions (see Figure 3b). Thec orresponding PL spectra show anarrow emission with asingle peak that is tunable from 400 to 690 nm depending on the halide composition (Figure 3c & Figure S9a). Interestingly,wefind that the red-shift of the PL peak while going from Cl to Br and Br to Ii sn early-linear with respect to their precursor ratio (see Figure S9b,cin the Supporting Information). This suggests that one can obtain ac olloidal solution with ar equired PL peak simply by adjusting the precursor ratio according to the linear fit. However,w eh ave to take into account that the exact halide composition of the NCs is not necessarily the same as the ratio of halides in the precursor solutions.Asshown in Figure 3d-g, the TEM images of the CsPbX 3 NCs with different halide compositions show nearly-monodisperse nanocubes regard-less of the halide composition (Figure 3d-g). Interestingly, the average size of the nanocubes decreases from~11.2 nm to 5.3 nm while going from It oC lv ia Br (Figure 3d-g,s ee Figure S10 and S11 in the SI for large area TEM images of CsPbI 3 and CsPbCl 3 NCs). This is likely due to the decrease in the size of the halide ion that reduces the lattice spacing as well as the differences in their nucleation. [3c, 7] Additionally,the versatility of this approach is verified by applying it to the synthesis of organic-inorganic hybrid perovskites,inparticular, relatively less explored formamidinium (FA) lead bromide (FAPbBr 3 )perovskite NCs by using FA-oleate instead of Cs-oleate (see the experimental section in the SI for details). Figure 4a shows the PL spectra of FAPbBr 3 NCs obtained at different reaction temperatures between 175 to 25 8 8C. Like in CsPbBr 3 NCs,t he PL peak of FAPbBr 3 also blue-shifts with decreasing the reaction temperature.I mportantly,t he spectra exhibit an arrow and single peak that is precisely tunable from~527 nm to 436 nm by varying the reaction temperature.The PL peak at 527 nm for FAPbBr 3 NCs generally,c orresponds to cubic morphology, which is also confirmed by TEM ( Figure 4b). As shown in Figure 4b,c ,t he FAPbBr 3 NCs prepared in the temperature range of 175-80 8 8Ce xhibit cubic morphology with their average edge-length decreasing from~8.1 nm to~5.1 nm (also see Figure S12 &S13 in the Supporting Information for large-area TEM images). It is worth mentioning that the exciton Bohr radius of FAPbBr 3 NCs was reported as 8nm. [16] Therefore,t he decrease in edge-length reflects the blue-shift

Angewandte Chemie
Research Articles of PL peaks from~527 nm to~499 nm due to the strong quantum-confinement of nanocubes obtained at 80 8 8C. [4c] However,u nlike CsPbBr 3 nanocubes,t he PLQY of nanocubes was found to be decreasing with the reduction in their size (see Table S2 in Supporting Information). Thepowder Xray diffraction pattern of the FAPbBr 3 nanocubes (both bulklike and 3D-quantum confined) resembles the cubic crystal phase reported in literature ( Figure S14). [17] However,further decrease of reaction temperature leads to the formation of FAPbBr 3 2D NPls,a ss hown in Figure 4d (see Figure S15 in the Supporting Information for large area TEM image of NPls). TheNPls exhibit excellent monodispersity and tend to form stacks on the TEM grid. ThePLpeak energy vs.the size/ thickness of FAPbBr 3 NCs obtained at different reaction temperatures,a long with their morphology,i si llustrated in Figure 4e.T he initial increase in PL energy of the NCs obtained by decreasing the reaction temperature from 200 to 50 8 8Cisdue to decrease of their size,suggesting the formation of strongly quantum confined 0D nanocubes.H owever,a t room temperature strongly quantum-confined 2D nanoplatelets with nonlinear increase in their PL energy can be seen. It is worth mentioning that 0D nanocubes and 2D NPls of FAPbBr 3 have been less explored compared CsPbBr 3 system. Our results demonstrate that this user-friendly hot-plate approach is very promising not only for inorganic perovskite NCs,but also for organic-inorganic hybrid perovskite NCs

Conclusion
In summary,wedemonstrated asimple,scalable,and userfriendly hot-plate approach for the shape-and size-controlled synthesis of halide perovskite nanocrystals in the air and using pre-synthesized precursor stock solutions.M ore importantly, we find that the reaction yields mainly nanocubes of different sizes (bulk-like and quantum confined) in the temperature range of 175 to 100 8 8C, however,f urther decreasing the reaction temperature leads to the formation of NPls.Surprisingly,h igher research temperature results in hexapod NCs with distinct optical properties compared to nanocubes due to their size differences.T he nanocrystals synthesized by this approach are as monodisperse as those prepared by classical hot-injection synthesis under an inert atmosphere,w ith as tandard deviation ranging from 10 to 15 %. Despite the synthesis in the air,the NCs exhibit narrow emission without applying any size-selective separation process.B esides,t he halide composition, and thus the PL emission, could be precisely tuned by using different ratios of corresponding PbX 2 (X = Cl, Br,o rI )p recursor solutions.I nterestingly,w e find that the PbX 2 precursorsr atio (PbBr 2 /PbCl 2 and PbI 2 / PbBr 2 )h as an ear-linear relationship with the PL peak position of the mixed halide perovskite nanocubes,t his enables the find an exact ratio to obtain NCs with as pecific PL emission. Importantly,t he versatility of the synthetic approach is demonstrated by applying it to less-explored FAPbBr 3 NCs.W edemonstrate the size and shape tunability from bulk-like 3D FAPbBr 3 nanocubes to strongly quantumconfined 0D FAPbBr 3 nanocubes with 3D-quantum confinement and 2D nanoplatelets with 1D-quantum confinement by the reaction temperature.I nterestingly,o nly below 50 8 8Ct he reaction yields FAPbBr 3 nanoplatelets.W es trongly believe that this user-friendly synthetic approach presenting here will not only be useful for the large-scale synthesis of lead halide perovskite NCs for device applications but also could be routinely used for the synthesis of various other perovskites NCs and their derivatives.